The published data on the essential amino acid requirements of man reveal a wide variability for the needs of certain of the individual essential amino acids among subjects of similar weight, age and sex. This variation is due to technical errors inherent in the design and methods of experiments conducted and in part to biological variation. More sensitive and rapid methods are required to quantitate precisely the needs and variations within the general population. There are significant gaps in our knowledge of the essential amino acid requirements in various age group, particularly elderly people. The practical significance of published estimates remains unknown. We propose to extend our use of the plasma amino acid response curve approach for assessing essential amino acid requirements of man. This method is potentially more accurate, sensitive and rapid than the nitrogen balance technique which previously has been the major criterion used to assess the minimal requirements for essential amino acids in man. Included in these studies will be a comparison of the essential amino acid needs in young adult men and elderly people, and an assessment of the longer term adequacy of the derived minimum requirements for the various essential amino acids. This proposal is modified from a similar previous proposal in two respects. 1) It will concentrate only on obtaining values based on the lower break-point since these are wholly consistent with those of Rose et al., Leverton et al. obtained from short term N balance studies in young adults. 2) It defers until a later stage, and separate proposal, plans for long-term N balance studies.